Crust-mantle mechanical coupling in Eastern Mediterranean and Eastern Turkey

Abstract

Present-day crust-mantle coupling in the Eastern Mediterranean and eastern Turkey is studied using the Global Positioning System (GPS) and seismic anisotropy data. The general trend of the shear wave fast-splitting directions in NE Turkey and Lesser Caucaus align well with the geodetic velocities in an absolute plate motion frame of reference pointing to an effective coupling in this part of the region of weak surface deformation. Farther south, underneath the Bitlis Suture, however, there are significant Pn delays with E-W anisotropy axes indicating significant lateral escape. Meanwhile, the GPS reveals very little surface deformation. This mismatch possibly suggests a decoupling along the suture. In the Aegean, the shear wave anisotropy and the Pn anisotropy directions agree with the extensional component of the right-lateral shear strains except under the Crete Basin and other parts of the southern Aegean Sea. This extensional direction matches perfectly also with the southward pulling force vectors across the Hellenic trench; however, the maximum right-lateral shear directions obtained from the GPS data in the Aegean do not match either of these anisotropies. Seismic anisotropy from Rayleigh waves sampled at 15 s, corresponding to the lower crust, match the maximum right-lateral maximum shear directions from the GPS indicating decoupling between the crust and the mantle. This decoupling most likely results from the lateral variations of the gravitational potential energies and the slab-pull forces.

Isotropic and Anisotropic P Wave Velocity Structures of the Crust and Uppermost Mantle Beneath Turkey

“…Beneath central northern Turkey where the dominant regime is transpressional, the NE-SW FVDs in the uppermost mantle are consistent with previous SKS FPDs (Figure 11). They are different from either the N-S FPDs in the lower crust or the E-W maximum shear direction derived from GPS measurements (Özeren, 2012;Özeren & Holt, 2010). These results imply that the crust and uppermost mantle are decoupled.…”

“…Recent advancements in both computational seismology and seismic observations have led to the development of 3-D high-resolution tomographic images including depth-dependent azimuthal and radial anisotropies in different tectonic regimes (e.g., Zhao, 2015), such as in subduction zones (e.g., Eberhart- Phillips & Henderson, 2004;Ishise & Oda, 2005;Wang & Zhao, 2008, 2012, continental regions (e.g., Huang & Zhao, 2013;Tian & Zhao, 2013;Wei et al, 2016), and a tectonically stable region (Munzarova et al, 2018). These extensive studies demonstrate that P wave anisotropic tomography is an effective and powerful technique to reveal 3-D variations of seismic anisotropy in the crust and mantle, since it provides improved vertical resolution compared with SWS measurements (e.g., Zhao et al, 2016).…”

Constraints on the Lithospheric Kinematics in the Aegean and Western Anatolia Unveiled by SKS Splitting Observations

“…If this is the fact, what is the possible geodynamic explanation of this phenomenon? Wang et al (2020) at the central part of the NAFZ have imaged evident decoupling between the crust and lithospheric mantle as the NE-SW fast P-wave directions in the uppermost mantle in their anisotropic P-wave local earthquake tomography model were correlated with previous SKS FPDs (e.g., Biryol et al, 2010;Paul et al, 2014), but different from either the N-S fast P-wave directions in the lower crust of their model or the GPS-derived E-W maximum shear directions (Özeren, 2012;Özeren & Holt, 2010). Beneath central Anatolia, a seismic low velocity zone (LVZ) is prevalent in the lower crust and uppermost mantle (between 50 km and >100 km), which gradually merges into the shallow Anatolian asthenosphere (e.g., Delph et al, 2015;Fichtner, Saygin, et al, 2013, Fichtner, Trampert, et al, 2013Wang et al, 2020).…”